Accurate knowledge of both antenna location and Radio Frequency (RF) feeder cable length between a base station and its antenna in a cellular radio system are necessary for the implementation of location based services.
The length of the feeder cable must be known because of the timing delay that it introduces and also to estimate the insertion loss of the feeder cable for the purposes of calculating appropriate transmit and receive power. It is possible to determine feeder cable length manually during cell site installation or upgrade, but this is time consuming, expensive, and prone to inaccuracy and human error.
Antenna location may be determined from an accurate map but this is unlikely to be accurate enough and information on height is likely to be poor. A stand-alone Global Positioning System (GPS) receiver can be used to give an accurate determination of location in three dimensions, but requires a long time to minimise errors. Alternatively, some known base stations incorporate a GPS receiver in the main body (e.g. cabinet) of the base station. An example of such a known cellular base station is shown in FIG. 1. The base station contains a high-accuracy (high-cost) GPS receiver 108, which is used as a reference for the base station's master oscillator 10 (which is itself used as a high precision timing reference for the base station 104). The length of the RF co-axial feeder cable 114 between the GPS antenna 112 and the GPS receiver 108 can be considered to be negligible. The main RF co-axial feeder cable 122 for cellular transmit and receive signals is much longer and is of a length that is not known accurately before installation.
After installation, feeder cable length is known to be measured using Time Domain Reflectometry (TDR), but the test equipment is expensive and requires a trained operator. Feeder cable length is also difficult to measure using other known methods, once the feeder cable is installed. Inaccuracy due to human error is also a serious concern.